Bringing Homogeneous Iron Catalysts on the Heterogeneous Side: Solutions for Immobilization

Abstract

Noble metal catalysts currently dominate the landscape of chemical synthesis, but cheaper and less toxic derivatives are recently emerging as more sustainable solutions. Iron is among the possible alternative metals due to its biocompatibility and exceptional versatility. Nowadays, iron catalysts work essentially in homogeneous conditions, while heterogeneous catalysts would be better performing and more desirable systems for a broad industrial application. In this review, approaches for heterogenization of iron catalysts reported in the literature within the last two decades are summarized, and utility and critical points are discussed. The immobilization on silica of bis(arylimine)pyridyl iron complexes, good catalysts in the polymerization of olefins, is the first useful heterogeneous strategy described. Microporous molecular sieves also proved to be good iron catalyst carriers, able to provide confined geometries where olefin polymerization can occur. Same immobilizing supports (e.g., MCM-41 and MCM-48) are suitable for anchoring iron-based catalysts for styrene, cyclohexene and cyclohexane oxidation. Another excellent example is the anchoring to a Merrifield resin of an Fe^II-anthranilic acid complex, active in the catalytic reaction of urea with alcohols and amines for the synthesis of carbamates and N-substituted ureas, respectively. A SILP (Supported Ionic Liquid Phase) catalytic system has been successfully employed for the heterogenization of a chemoselective iron catalyst active in aldehyde hydrogenation. Finally, Fe^III ions supported on polyvinylpyridine grafted chitosan made a useful heterogeneous catalytic system for C-H bond activation.

Keywords: MCM-41; SILP; alkene/alkane oxidation; carbamate synthesis; chitosan; hydrogen transfer; immobilization; ionic liquid (IL); iron catalysts; olefin polymerization.

Figure 1

Crystal structures of (a) [Fe(^iPrPDI)(N₂)₂] (CCDC: 254793) [18], (b) the Knӧlker catalyst [Fe(η⁵-L)(CO)₂(H)], L = 1,3-bis(trimethylsilyl)-4,5,6,7-tetrahydro-2H-inden-2-ol (CCDC: 114304) [19], and (c) [Fe(L’)(CH₃CN)₂](BF₄)₂ (L’ = (R,R)-N,N’-bis(2-(diphenylphosphino)ethylidene)-1,2-diphenylethylene-diamine (CCDC: 728131) [20], BF₄ anions omitted for clarity. Structures reproduced with Mercury 4.3.1 [21], color code: Fe = orange, P = green, Si = yellow, O = red, N = blue, C = grey, H = white.

Figure 2

(a) The two-step synthesis of the iron pre-catalysts with bis(arylimino)pyridyl ligands for polymerization of olefins and (b) different combinations of R¹, R², R³ and R⁴ groups tested in complexes 1–8 [45].

Figure 3

Herrmann’s functionalization of the iron pre-catalyst in three steps and immobilization on modified silica surface using the platinum-based Karstedt catalyst [47].

Figure 4

Kim’s immobilization strategy over silica for iron pre-catalyst [50]. Conditions: (i) EtOH, concentrated H₂SO₄, 90 °C; (ii) K₂CO₃, allyl bromide, acetone, reflux; (iii) 5 eq L⁻¹ NaOH, THF, 50 °C; (iv) SOCl₂, DMF, 90 °C; (v) CuI, MeLi, Et₂O, THF, −78 °C; (vi) 2,6-dimethylaniline, EtOH, AcOH, reflux; (vii) (CH₃)₂SiHCl, H₂PtCl₆⋅6H₂O (catalyst), CH₂Cl₂, EtOH:Et₃N 1:1; reflux; (viii) silica gel, toluene, 120 °C.

Figure 5

Li’s immobilization strategy over silica for iron pre-catalyst [51].

Figure 6

Immobilization of iron pre-catalyst performed by Jin and Liu through copolymerization with polystyrene (PS) [52].

Figure 7

Immobilization of 2,6-bis[1-(2,6-diisopropylphenylimino)ethyl]pyridyl dichloro iron(II) on MCM-41 [54].

Figure 8

Immobilization of FePcS on the inner surface of functionalized mesoporous MCM-48 and MCM-41 silica substrates [55].

Figure 9

The stepwise preparation of MCM-Py-Fe(III) from MCM-41, [Fe] = anchored complex [57].

Figure 10

Structure of the functionalized polymeric resin realized by Merrifield in 1963 [70].

Figure 11

Synthesis of the catalyst [Fe^II(Antra-Merf)] immobilized over a PS Merrifield resin [72].

Figure 12

Reaction between urea and different alcohols with the catalyst [Fe^II(Antra-Merf)] yielding carbamates [72].

Figure 13

Reaction scheme between urea and differently substituted amines with the catalyst [Fe^II(Antra-Merf)] yielding N-substituted ureas [72].

Figure 14

Proposed reaction mechanism between urea and alcohols in the presence of the catalyst [Fe^II(Antra-Merf)] (Antra-Merf = O^N) [72].

Figure 15

(a) General formula for metal complexes with ‘pincer’ tridentate ligands; (b) synthesis of the complex [Fe(PNP-Me-iPr)(CO)(H)(Br)] performed by Kirchner et al. as mixture of isomers, where the active species in catalytic hydrogenation reaction of C=O double bonds is the one with H in trans to Br [92].

Figure 16

Hydrogenation mechanism of acetaldehyde proposed by Kirchner et al. (DBU = 1,5-diazabiciclo(5.4.0)undec-7-ene) [93].

Figure 17

Structure of the SILP catalytic system exploited by Kirchner et al. with [bm₂im][NTf₂] as IL anchored on silica particles and [Fe(PNP-Me-iPr)(CO)(H)(Br)] as pre-catalyst (CAT) [94].

Figure 18

Synthesis of chitosan by partial deacetylation of chitin.

Figure 19

(a) Chitosan polymeric chain grafted with polyvinylpyridyl pendants; (b) Oxidation reaction of methylpyridazone using the grafted chitosan with immobilized Fe³⁺ ions as catalyst (Ar = phenyl, 4-chlorophenyl) [108].

Figure 20

(a) Chitosan polymeric chain grafted with 2-cyano-1-(pyridin-3-yl)allyl acrylate (CPA); (b) Proposed mechanism of the oxidation reaction of methylpyridazone using grafted chitosan with immobilized Fe³⁺ ions (Cs–Fe³⁺) as catalyst (Ar = phenyl, 4-chlorophenyl) [111].